Diesel engines, like most engines, can require compliance with a variety of emission standards. In order to meet emission standards, the exhaust gases exiting the diesel engine can be treated by a variety of treatments to remove or minimize unwanted aspects. An exhaust system is generally associated with a diesel engine system for treating exhaust gases to meet emission standards.
Exhaust systems for diesel engines can treat and reduce oxides of nitrogen (NOx) present in exhaust gas flow, prior to the exhaust gas flow exiting into the atmosphere. For example, an aqueous urea solution made from certain percentages of urea and deionized water, such as Diesel Exhaust Fluid (DEF) (e.g., 32.5% urea and 67.5% deionized water), collectively now DEF, can be injected into exhaust gases as the exhaust gases flow through a portion of the diesel exhaust system to reduce NOx emissions. Water from the DEF typically evaporates first, then the urea undergoes a thermolysis process where it breaks down to isocyanic acid (HNCO) and ammonia. The HNCO can react with water to form another NH3 (and carbon dioxide, CO2). However, once the water evaporates the urea can react with itself and HNCO to form unwanted byproducts, such as biuret. The biuret can further react to produce cyanuric acid, ammelide, and ammeline, which may deposit in the diesel engine exhaust system. The rate of DEF-based deposit formation is a function of the time urea is in contact with itself and HNCO, which is dependent on the exhaust temperature and flow rate, DEF dosing rate, etc.
Thus, injection of DEF can form a film on surfaces of the diesel engine exhaust system that, if not suitably removed, can lead to unwanted deposit formations at various portions of the exhaust system. The unwanted deposit formations can be difficult to remove and can also lead to reduction in fuel efficiency, filter failure, damage or blockage to an SCR catalyst, and excessive back pressure, for instance. Additionally, the DEF can form a film in or on a hydrolysis catalyst, if such a catalyst is employed in the exhaust system, causing accumulation of urea, which can adversely impact DEF dosing control.
One way to reduce unwanted deposit formations is to increase temperature of the exhaust gases, for instance, at a location where the DEF is injected. For example, a Continuous Regeneration System (CRS) can be used to increase exhaust temperature. Another way to increase exhaust temperature is to perform an in-cylinder injection of diesel fuel at the end of each combustion cycle. However, such methods can increase fuel consumption or are impractical. Another way to reduce deposit risk is by way of a hydrolysis catalyst. However, as noted above, DEF can form a film in or on the hydrolysis catalyst causing accumulation of urea, which can adversely affect DEF dosing control.
U.S. Patent Publication Number 2011/0030350 (hereinafter “the '350 publication”) describes an exhaust gas purification apparatus having an oxidation catalyst layer that supports on a downstream end surface thereof a hydrophilic layer having a hydrophilic function and forming a urea decomposition accelerator. According to the '350 publication, the hydrophilic layer is formed by coating the end surface of the oxidation catalyst with a catalytic material that has a hydrolytic catalytic function for accelerating the hydrolysis and a hydrophilic function. The '350 publication also describes injecting urea water toward a downstream surface of the hydrophilic layer.